For subjects with presbyopia, the lens of the invention seeks to compensate for any heterophoria that exists in addition to the presbyopia. Heterophoria, i.e. departure of the two visual axes from the point of fixation under consideration, remains latent in the absence of any appropriate fusion stimulus.
Consequently, for this particular fixation distance, the active and passive positions do not coincide. Depending on the relative direction of the deflection, various forms of heterophoria are described: esophoria (inward deflection of the visual axes of the eyes), and exophoria (outward deflection of the visual axes of the eyes). (Source: “Le Nouveau Dictionnaire de la Vision” [The new dictionary of vision] by Michel Millodot, published by Mediacom Vision 1997, Bures sur Yvette, p. 72.). Most individuals present heterophoria to a greater or lesser extent. To compensate for this divergence or convergence of fixation lines, the subject needs to converge or diverge to a greater extent. Depending on the subject, the subject's state of fatigue, or the subject's age, this additional converging effort can lead to visual discomfort, particularly for near vision. If compensation is insufficient or too laborious, heterophoria can even turn out to be an obstacle to wearing progressive lenses.
For subjects who are myopic or emmetropic, the purpose is to control progress of myopia, and in particular to prevent or slow down the progress of myopia in children. The modern world is pushing an increasing that involves intensive use of vision that is near, intermediate, and central. Working with near vision thus occupies a major place in the daily activities of the visual system of a child or an adult. It would appear that working with near vision plays a role in the development of myopia. Recent studies confirm the importance of this environmental factor, particularly with children. The level of school attendance seems to have an influence on the percentage of people with myopia in a given population, and the degree of myopia appears to be correlated with the number of hours spent each day using near vision. In western countries, the proportion of people with myopia lies in the range 15% to 20% This proportion rises to 50% or even 70% in Asian countries such as China and Japan. In African countries, the proportion of people with myopia (other than those with congenital myopia) is down to practically zero.
It has thus been put forward that acquired myopia (in contrast to congenital myopia that is present at birth) is associated with prolonged work with near vision, and that the level of myopia depends on the length of time spent working with near vision. Myopia would thus be an adaptation of the visual system to working with near vision. Optometrists have thus proposed solutions attempting to slow down the progress of acquired myopia. A first solution consists in prescribing glasses that correct a wearer's myopia by an amount that is less than that person's myopia in far vision, the idea being to reduce the accommodation effort needed to go from far vision to near vision. That method is open to dispute and has not been proved to be effective. Another solution consists in prescribing correction that is too great for a person's far vision. The ametropia of young sufferers of hypermetropia is more stable over time than is the ametropia of young sufferers of myopia, and the idea is to make young sufferers of myopia hypermetropic by prescribing them with too much correction. The effectiveness of that method has not been demonstrated either. Other specialists have proposed visual training: repeated visual experiments of relaxation and exercises for relieving tension in the eyes. The effect of those methods on the ametropic of the subject have not been proved.
Finally, optometrists have attempted to slow down progress of myopia by prescribing the subject with a spherical addition of power in near vision. Such an addition would have the effect of reducing the effort of accommodation in near vision. In order to verify that hypothesis, studies have been carried out on prescribing bifocal lenses or lenses with progressive addition of power to children with myopia. Those studies are generally incomplete and have given mixed results on the progress of myopia: effectiveness is found to vary depending on the wearer under test, such that it has not been possible to put forward any valid solution for systematic and effective treatment of all or at least the majority of the childhood population with myopia.